Our invention relates to a system for, and a method of, driving an electric stepping motor such as that employed, for example, in a disk drive for feeding a magnetic transducer head assembly, or a pair of such assemblies, radially of a flexible magnetic disk so as to enable the head assembly or assemblies to access the individual tracks on one or both sides of the disk.
Bidirectional stepping motors have found widespread use in the radial feed mechanism for the head assembly or assemblies of a disk drive, as disclosed for example in Dalziel et al. U.S. Pat. No. 3,678,481. The stepping motor in general has one or more sets of several phase windings arranged sequentially on stator poles of annular arrangement around a rotor which may be of permanent magnet material. The sequential excitation of the stator windings results in the rotation of the rotor in discrete increments or steps. Despite its simplicity of construction and ease of control, the stepping motor makes it possible to rather accurately position the transducer head assembly or assemblies on desired tracks on the magnetic disk.
We said "rather accurately" because, upon cessation of the excitation of the stator windings for setting the stepping motor out of rotation, the rotor may not come into exact alignment with the required stator poles if the load on the motor is comparatively heavy. The reason for this is the insufficiency of the attractive forces exerted electromagnetically by the required stator poles upon the rotor poles of opposite polarity. In such cases, even if the same stator windings are finally energized for setting the rotor out of rotation in a position of alignment with the associated stator poles, the rotor will come to a stop in different angular positions slightly out of alignment with the required stator poles depending upon the direction in which it has been in rotation. Such differences between the stopped positions of the rotor, known to the specialists as hysteresis errors, present a serious problem, for example, in the radial positioning of the transducer head assembly or assemblies on the magnetic disk.
In solving the above problem we should give heed to the fact that the attractive force exerted by each stator pole on the rotor is weaker when the rotor pole of opposite polarity is in exact alignment therewith than when it is slightly out of alignment in either direction. There is, in short, an "insensitive zone" associated with each stator pole, where the stator is more or less incapable of magnetically controlling the rotor.